okay, thanks for all the suggestions.. I forgot to include that i have a new rad cap as of this weekend as well. I did the burp thing this morning just to see. My driveway is fairly steep so i parked with the front raised and started the engine and let it warm up. I also turned on the heater but i believe the hoses that feed the heat coil are always on even when the thermostat is closed its still flowing. No bubbles, but the coolant warms up and expands out the open lid and blows all over the place. Also the fan shroud is in place.

So does the boiling into the reservoir give a positive indication of overheating. because it only happens after i shut the motor off. Makes me think that either 1. Since the fan turns off with the motor, it is sending the temperature of the coolant above boiling. and or 2. pressure is being released somehow and causing the boiling temp to lower.

When the jeep starts overheating i do not get a check engine light or any type of warning and i dont notice the engine running differently. That and the fact that it happens so suddenly is what is making me think something is wrong with whatever is reporting the temp.

not that i can tell. ive been burping it so i loose a bit there but nothing noticeable

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have you tryed back flushing the system to make sure nothing is in there blocking it up

i havent because everything is fairly new, coolant looks perfect, inside the radiator everything still looks pristine. heater core was brand new not that long ago.

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auto or 5sp? Any obstructions in front of the radiator other than the ac condenser?

Automatic with AC and the ac radiator looks as dirty as any other car ive ever owned.. a lot of dead bugs and such. How the heck do i clean that thing anyways.. i hit it with the spray nozzle this weekend while washing it and didnt seem to dislodge anything.

I feel like the next thing is to replace the hoses. Ive heard you can have the system pressure checked for leaks.. and some other test to see if coolant is getting into a cylinder. sound about right?

when you turn on your heat coolant goes in to the heater core so it will push out any air that could be in there.

The TJ does not use a heater valve. Hot coolant is always circulating through the heater hoses and heater core. The only thing that keeps the hot air from mixing with the vent or AC is the blend door.

The heater hoses are also the engines coolant bypass. Do not plug them off. If you need to remove them from the heater to stop a leaking heater core you need to loop them so coolant can bypass when the thermostat is closed.

head gasket sounds like it shouldn't be to difficult to fix, but the cracked head scares me. The jeep drives fine otherwise. This pas week has been cool so no issues at all. Whats the best way to verify the leaking head gasket or cracked head?

With everything you've done you must have some type of block in there somewhere. I'm not sure if this is proper but I would find a way to reverse flush the thing with a garden hose or something and see what happens. And a blocked radiator would (most likely) not cause an overheat at idle, not much air moving through it anyway. When I say blocked I don't mean liquid block I mean air movement.

First warm it up real good, shut it off and check the oil cap and dipstick looking for muddy looking oil, it should be pretty obviously brownish green if it's leaking bad. However you may have a blown gasket and not have these symptoms

If nothing then start pulling spark plugs looking for coolant on them

I've heard some people test it with all plugs out except one, pull off the radiator cap as well and crank the engine, if you see bubbles the gasket is blown (I can't verify this method).

Some people will do compression tests or leakdown tests, but those can easily miss a blown head gasket

The best test (other than seeing obvious signs of coolant in the oil) is to use a chemical test kit, you should be able to find one doing a google search for head gasket test kit. It has a test tube sort of thing that sits on the radiator in the fill spout, and has a color changing fluid that lets you know if engine gasses are entering the coolant.